1. Field of the Invention
This invention relates to a needle thread control device for handling needle threads by taking up and supplying needle threads between thread guides, in a sewing machine, or particularly an overedge sewing machine used for forming stitches of stitch types 503, 504, 514, 516 specified in Federal Standard No. 751a.
2. Prior Art
In the type of overedge sewing machine discussed above, generally, a needle thread handling means is disposed on the path for leading the needle threads from the thread supply source to the sewing needles, and the looseness of the needle threads is absorbed when the needles go up, and the stitches are tightened and the needle threads are pulled off from the needle thread supply source, while the threads are tightened while the needles descend so as to compensate the threads necessary for forming needle thread loops.
However, in the case of, for example, double chain stitches (stitch type 401) which is a part of stitch type 516, when the needle falls into the triangle formed by the looper, the needle thread loop captured by the looper, and the looper thread, if the thread is too loose, the needle thread loop captured by the looper may be tilted or deviated, and the necessary triangle may not be formed. As a result, the needle may hook the needle thread loop or pass over the outside of the triangle to cause thread breakage or skipping of stitches. To solve these problems, it is proposed in Japanese Patent Publication No. 55-43797 to dispose clamp means between the needle thread handling means mounted on the needle bar and the thread guide mounted on the sewing machine main body, to prevent slackening of the needle threads by clamping the needle threads within the clamp means and varying the elevation of the needle thread handling means until the needles fall into the triangle. In Japanese laid-open Utility Model No. 60-188581 it is proposed to install a first thread control cam on the needle bar and the second thread control cam in the lever on which the cloth cutting knife is mounted, so as to absorb the looseness of the thread caused by the descending action of the needle and the action of both thread control cams on the needle threads until the needles fall into the triangle.
Nevertheless, since the needle thread handling means and the thread control cams cannot be placed into the sewing machine frame in an enclosed structure, such means cannot be placed on the needle bar mechanism of a high speed overedge sewing machine having an enclosed frame and lubricated needle bar mechanism in the frame.
Meanwhile, when the draw of the needle thread is short, the stitches are taut, and the soft touch is lost. Such problem occurs not only in the double chain stitches but also in overedge stitches. Therefore, in the overedge sewing machine, generally, the mounting position of the needle guide is adjustable and the position where stitches of soft touch are formed for typical cloth and thread is regarded as the reference position, and the mounting position is adjusted depending on the kind of cloth and thread, and the thread draw is adjusted by the needle thread handling means. When the mounting position is adjusted, the thread draw amount and the thread handling amount by the needle thread handling means will vary significantly, which makes it difficult to obtain stitches of desired touch and look. Furthermore, the reference position was conventionally based on the throat plate or front face of the sewing machine, and placed at the distance from that position to the eyelet of the thread guide, and accordingly, the placement was not easy, because the measurement of the distance was essential to the placement.